For many polyolefin applications, including films and fibers, increased melt strength and good optical properties are desirable attributes. Higher melt strength allows fabricators to run their blown film lines at a faster rate. It also allows them to handle thicker films in applications such as geomembranes.
Typical metallocene catalyzed polyethylenes (mPE) are somewhat more difficult to process than low-density polyethylenes (LDPE) made in a high-pressure polymerization process. Generally, mPEs (which tend to have narrow molecular weight distributions and low levels of branching) require more motor power and produce higher extruder pressures to match the extrusion rate of LDPEs. Typical mPEs also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and are prone to melt fracture at commercial shear rates. On the other hand, mPEs exhibit superior physical properties as compared to LDPEs. In the past, various levels of LDPE have been blended with the mPE to increase melt strength, to increase shear sensitivity, i.e. to increase flow at commercial shear rates in extruders; and to reduce the tendency to melt fracture. However, these blends generally have poor mechanical properties as compared with neat mPE. It has been a challenge to improve mPEs processability without sacrificing physical properties.
U.S. Publication No. 2007/0260016 discloses blends of linear low density polyethylene copolymers with other linear low density polyethylenes or very low density, low density, medium density, high density, and differentiated polyethylenes, as well as articles produced therefrom. U.S. Publication No. 2007/0260016 does not appear to disclose means to obtain a balance of improved processability and physical properties.
Guzman, et al., AIChE Journal, May 2010, Vol. 56, No. 5, p. 1325-1333 discloses ethylene/octene/1,9-decadiene copolymers and a method to predict gel formation in the production thereof. The publication is silent on the technical features that would be needed to make the decadiene terpolymer suitable for providing the optimum balance of processability and physical properties.
U.S. Pat. No. 6,300,451 discloses ethylene/butene/1,9-decadiene copolymers, and ethylene hexene vinyl norbornene copolymers (see Tables I and II). The decadiene terpolymers disclosed are designed to be used alone and not in blends for improved processability/property balance. The relatively high MI of the resins suggests that they would not be suitable in blends which exhibit improved extensional strain hardening.
U.S. Pat. No. 5,670,595 discloses diene modified polymers, particularly diene modified propylene polymers that would not be suitable for modification of polyethylene based polymers due to their incompatibility.
U.S. Pat. No. 6,509,431 discloses ethylene/hexene/1,9 decadiene copolymers. The low melt index ratio of the disclosed polymers suggests that they would not be suitable for rheology modification (increased strain hardening) of the base linear polyethylene.
Further, one of challenges for cast film process is the neck-in phenomenon. Neck-in is the reduction of cast film width from die to final dimension. It is primarily related to relaxation time of the polymer. The branched modifiers of this invention are very effective for reducing the neck-in, as compared to LDPE, which is conventionally used for this purpose. Addition of the branched modifier(s) into cast grade LLDPE resin significantly minimizes the neck-in. The effectiveness of branched modifiers allows a low level addition which in turn will retain the physical properties of the base resin.
Other references of interest include: U.S. Pat. Nos. 7,687,580; 6,355,757; 6,391,998; 6,417,281; 6,114,457; 6,734,265; 6,147,180; and PCT Publication Nos. WO 2007/067307; WO 2002/085954.
We have discovered that certain branched hydrocarbon modifiers, preferably comprising dienes, will advantageously improve processability, without significantly impacting its mechanical properties, such as dart impact. Moreover, addition of these branched hydrocarbon modifiers provides a means to change such properties on a continuous scale, based on real-time needs, which is typically not possible due to the availability of only discrete polyethylene grades. Furthermore, a different set of relationships between processability and properties is obtained, compared to those available from traditional polyethylenes and their blends with conventional LDPE, which allows for new and advantageous properties of the fabricated articles.